15mm height would not fit, no.
2.5 inch width yes; as long as the SSD interface is the same of course and the height does not exceed say 5mm.
This really depends on what software you are using but a 7200rpm drive should be fine. Something like 4K video might be an issue, but you should totally be fine with audio.
Yes, there is room due to the excellent anti shock gap/space and foam.
Pay attention though, a larger tighter fit will reduce shock mitigation. There would be less space to take up room for movement when dropped.
Interestingly, the mid 2011 iMac 27 inch also goes to 12GB!
I originally thought I'd be upgrading to 8GB today but found 4 slots instead of 2 :-)
As long as the two front (close to the screen) slots are paired with the exact same RAM config.
Two front slots Apple 2GB chips = 4GB.
Two rear slots, Crucial Lexar 4GB chips = 8GB
Hoorah no waste and more RAM, come on Lion... I'm Ready!
Regarding the following instruction: -
Two 3.3 mm silver Phillips screws on either side of the SuperDrive.
Be aware there are two of these on the bottom (if the slot is facing you) screws close to each other, one is for the actual drive assembly itself and the other is attaching the drive to the Mac. Beware you choose the correct one.